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Earth and Planetary Science Letters,
71 (1984) 229-24 0 229
Elsevier Science Publishers B.V., Am sterdam - Printed in The Netherlands
[3]
h e s t a b l e i s o t o p i c c o m p o s i t i o n o f m o d e r n s o i l c a r b o n a t e a n d i t s
r e l a t i o n s h i p t o c l i m a t e
T h u r e E C e r l in g
Department of Geology and Geophysics, University of Utah, Salt Lak e C ity, UT 84112 U.S.A.)
Received January 4, 1984
Revised version received August 29, 1984
The oxygen isotopic composition of mo dern soil carbonate is well correlated w ith the isotopic composition of local
meteoric water. The carbon isotopic cycle for CO 2 in soils can b e described in terms of the prop ortion of biomass using
the C4 photosynthetic pathway and the CO 2 respiration rate o f the soil; at low soil respiration rates significant
atmospheric CO2 mixing can occur. In general, the carbon isotopic composition of soil carbonate is related to the
propor t ion of C4 biomass present in soil , but soils that freeze to the depth of carbonate formation often have a
significant atmospheric component. This suggests that freezing of the soil solution should be considered as another
imp ortan t mechanism for soil carbonate formation. Bec ause of these relationships, the isotop ic composition of soil
carbonate may be a paleoclimatic and paleoecologic indica tor in cases in which diagenetic alteration has no t occurred.
1 I n t r o d u c t i o n
S o i l c a r b o n a t e f o r m s u n d e r a r i d t o s u b - h u m i d
c l i m a t i c c o n d i t i o n s [ 1, 2] . I n g e n e r a l , i t is f o u n d i n
r e l a t iv e l y d r y s o i ls w h e r e g r a s s e s o r m i x e d g r a s s e s
a n d s h r u b s a r e t h e d o m i n a n t v e g e t a t io n . U n d e r
t h e s e c o n d i t i o n s s o i l p H i s g e n e r a l l y 7 o r a b o v e , i n
c o n t r a s t t o f o r e s t e d s o i ls w h e r e p H i s b e l o w 6 .
A u t h i g e n i c s o i l c a r b o n a t e i s c o m m o n i n s oi ls w h e r e
m e a n a n n u a l r a i n f a l l is l e s s t h a n 7 5 c m w h i l e i t i s
r a r e l y f o u n d i n s o il s r e c e i v in g m o r e t h a n 1 0 0 c m
p r e c i p i t a t i o n p e r y e a r .
S t a b l e i s o t o p e s i n s o i l c a r b o n a t e s a r e u s e f u l
t r a c e r s o f t h e i n f l u e n c e o f c l i m a t e o n s o i l - f o r m i n g
p r o c e s s e s . O x y g e n i s o t o p e s i n m e t e o r i c w a t e r s a r e
r e l a t e d t o c li m a t e , e s p e c i a ll y m e a n a n n u a l t e m p e r -
a t u r e [ 3 - 5 ]. T h e c a r b o n i s o t o p i c c o m p o s i t i o n o f
s o i l C O 2 d u r i n g t h e g r o w i n g s e a s o n i s r e l a t e d t o
t h e c a r b o n i s o t o p i c c o m p o s i t i o n o f th e b i o m a s s
[ 6 - 9 ] w h i c h i s r e l a t e d t o t h e p r o p o r t i o n o f p l a n t s
t h a t u s e t h e C 4 p h o t o s y n t h e t i c p a t h w a y [ 1 0, 11 ] .
T h e f l u x o f w a t e r i n a s o i l r e c e i v i n g 5 0 c m o f r a i n
p e r y e a r i s 2 .8 m o l e s c m - 2 y - l ; t y p i c a l f lu x e s o f
b i o g e n ic C O f r o m g r a s s l a n d s o i l s a r e o n t h e o r d e r
o f 5 1 0 - 3 m o l e s c m 2 y - a [ 12 ,1 3 ]. B e c a u s e s o i l
c a r b o n a t e f o r m s a t m u c h l o w e r r a te s t h a n t h i s ,
t y p i c a l l y 1 10 6 t o 1 10 - 5 m o l e c m - 2 y - 1
[ 1 4 - 1 6 ] , i t i s l i k e ly th a t t h e o x y g e n a n d c a r b o n
i s o t o p i c c o m p o s i t i o n o f s o il c a r b o n a t e i s c o n -
t r o l l e d b y t h e o x y g e n i s o t o p i c c o m p o s i t i o n o f
m e t e o r i c w a t e r s a n d b y t h e c a r b o n i s o t o p i c c o m -
p o s i t i o n o f s o i l C O 2 , r e s p e c t i v e l y .
F e w p r e v i o u s s t u d i e s h a v e e x a m i n e d t h e is o -
t o p i c c o m p o s i t i o n o f s o i l c a r b o n a t e ( e .g ., [ 1 7 - 2 2 ] ) .
T h e r e l a t i o n s h i p b e t w e e n t h e i s o t o p i c c o m p o s i t i o n
o f m e t e o r i c w a t e r a n d s o il c a r b o n a t e a n d b e t w e e n
v e g e t a t i o n a n d s o il c a r b o n a t e h a s n o t b e e n e s t a b -
l i s h e d b y t h e s e s t u d i e s b e c a u s e t h e y h a v e e x -
a m i n e d c a r b o n a t e s f o r m e d o n l i m e s t o n e p a r e n t
m a t e r i a l [ 1 7 ,1 9 , 21 , 2 2] o r h a v e s t u d i e d s o i l
c a r b o n a t e s f o r m e d d u r i n g s e v e r a l c l i m a t i c e p i s o d e s
o r d u r i n g u n k n o w n c l i m a t i c c o n d i t i o n s [ 1 7 - 2 2 ] .
T h i s s t u d y e x a m i n e s s o il c a r b o n a t e s f o r m e d d u r i n g
a b o u t t h e p a s t 1 0 , 0 0 0 y e a r s ; i t is o n l y f o r th i s t i m e
i n t e r v a l t h a t o n e c a n u s e t h e m o d e r n v a l u e s f o r th e
i s o t o p i c c o m p o s i t i o n o f m e t e o r i c w a t e r s a n d t h e
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m o d e r n v e g e t a t i o n a s a n e s t i m a t e o f t h e i s o t o p i c
c o m p o s i t i o n o f s o il w a t e r a n d s o il C O 2 t h a t p r o -
d u c e d t h e s oi l c a r b o n a t e s e x a m i n e d . I n a d d i t i o n ,
s o m e r e s u l t s o n p a l e o s o l c a r b o n a t e s a r e r e p o r t e d
b e c a u s e p a l e o s o l c a r b o n a t e s m a y g i v e a n i n d i c a -
t i o n o f a n c i e n t c l i m a t i c o r e c o l o g i c c o n d i t i o n s .
2 T e r m i n o l o g y a n d m e t h o d s
I n o r d e r t o u s e s t a b l e i s o t o p e s t o s t u d y
c a r b o n a t e f o r m a t i o n a n d i ts r e la t i o n s h ip t o c li m a t e ,
i t i s n e c e s s a r y t o e l i m i n a t e t h e p r o b l e m o f
c a r b o n a t e n o t f o r m e d i n t h e s o i l . I n t h i s s t u d y
o n l y t h o s e s o i l s o r p a l e o s o l s w e r e s t u d i e d w h e r e
m a r i n e l i m e s t o n e m a k e s u p a n e g l i g i b l e f r a c t i o n o f
t h e p a r e n t m a t e r i a l . T h r o u g h o u t t h i s p a p e r , t h e
t e r m so i l carbonate i s u s e d t o r e p r e s e n t o n l y t h e
c a r b o n a t e i n t h e s o il o r p a l e o s o l t h a t i s a u t h i g e n i c ,
t h a t i s , f o r m e d i n p l a c e . Calcre te s r e p r e s e n t a
s p e c i a l s o r t o f s o i l c a r b o n a t e d e p o s i t : t h a t o f m a s -
s iv e , c o n t i n u o u s h o r i z o n s m e a s u r i n g u p t o s e v e r a l
m e t e r s i n t h i c k n e s s . T h e y a r e g e n e r a l l y f o u n d i n
r e g i o n s t h a t r e c e i v e f r o m 2 5 t o 7 5 c m a n n u a l
r a i n f a l l .
M o d e r n s o il c a r b o n a t e s f r o m A f r i c a a n d N o r t h
A m e r i c a w e r e a n a ly z e d . S a m p l e s w e r e c h o s e n f r o m
a r e a s w h e r e i t w a s p o s s i b l e t o e s t i m a t e t h e p r o p o r -
t i o n o f C 4 b i o m a s s p r e s e n t w h e n s oi l c a r b o n a t e
f o r m a t i o n o c c u r r e d . S a m p l e s s t u d i e d a n d t h e i r l o -
c a l i ti e s a r e b r i e f ly d e s c r i b e d i n A p p e n d i x 1 .
C a r b o n a t e s w e r e r e a c t e d w i t h 1 00 p h o s p h o r i c
a c i d t o l i b e r a t e C O 2 . M o d e r n s o il c a r b o n a t e s w e r e
r o a s t e d a t 4 5 0 C u n d e r v a c u u m p r i o r t o r e a c ti o n
w i t h H 3 P O 4 . O x y g e n a n d c a r b o n i s o to p i c r a t i o s
a r e r e p o r t e d i n t h e s t a n d a r d n o t a t i o n r e l a t i v e t o
t h e P D B s t a n d a r d . W a t e r s a m p l e s d i s cu s s e d in t h e
t e x t w e r e a n a l y z e d b y e q u i l i b r a t i o n w i t h C O 2 g a s
a n d s u b s e q u e n t m a s s s p e c t r o m e t e r a n a ly s i s; t h e y
a r e r e p o r t e d r e l a t i v e t o t h e i s o t o p i c s t a n d a r d
S M O W . O r g a n i c c a r b o n f r o m t w o s o il s w a s
a n a l y z e d b y c o m b u s t i o n a t 8 0 0 C u s i n g C u O a n d
A g f o i l [3 0] a f t e r C a C O 3 w a s r e m o v e d w i t h 1 0
H C 1 .
3 R e s u l t s a n d d i s c u s s i o n
T h e r e s u l t s o f a l l i s o t o p i c a n a l y s e s o f m o d e r n
s o i l c a r b o n a t e a r e g i v e n i n T a b l e 1 . I n a d d i t i o n ,
e s t i m a t e s o f t h e f r a c t i o n o f C 4 f l o r a a n d o f t h e
i s o t o p i c c o m p o s i t i o n o f m e t e o r i c w a t e r a r e i n -
c l u d e d f o r e a c h s i t e .
3 . 1 . Re la t ionsh ip be tween the oxygen i so top ic com
p o s i t io n o f so i l c a r b o n a t e a n d m e t e o r i c w a t e r
I t h a s p r e v i o u s l y b e e n s u g g e s t e d t h a t t h e o x y g e n
i s o t o p i c c o m p o s i t i o n o f s o i l c a r b o n a t e m a y b e
r e l a te d t o t h e o x y g e n i s o t o p i c c o m p o s i t i o n o f
m e t e o r i c w a t e r [2 0 ]. S a l o m o n s e t a l . [ 20 ] f o u n d
p o o r a g r e e m e n t b e t w e e n t h e e x p e c t e d 6 1 ~ O v a l u e s
o f s o il c a r b o n a t e c a l c u la t e d f r o m e s t i m a t e d
m e t e o r i c w a t e r c o m p o s i t i o n s a n d t h e m e a s u r e d
6 1 8 0 v a l u e s o f s o i l c a r b o n a t e . T h e y a t t r i b u t e d t h i s
o b s e r v a t i o n t o d i f f e r e n c e s i n m e c h a n i s m s o f s o i l
c a r b o n a t e f o r m a t i o n . H o w e v e r , b e c a u s e t h o s e
c a l c r e t e s f o r m e d o v e r m u c h o f t h e p a s t m i l l i o n
y e a r s , it is p o s s i b l e t h a t t h e y f o r m e d u n d e r c l i m a t i c
c o n d i t i o n s d i f f e r e n t f r o m t h e p r e s e n t s o t h a t m o d -
e r n i s o t o p i c v a l u e s o f m e t e o r i c w a t e r s h o u l d n o t b e
a p p l i e d t o t h o s e e x a m p l e s .
S a m p l e s i n t h i s s t u d y w e r e c h o s e n t o m a x i m i z e
t h e r a n g e o f 6 1 8 0 o f m e t e o r i c w a t e r ; F i g . 1 s h o w s
0
b J
Z
0
m - 5
n , -
.
r e sp e c t i v e l y . T h e se a r e :
c ? = c i * R ,
] fo r R a a n d R s
a n d :
R,
l l + R i ] fo r R~,
w h e r e R S, R , , a n d R a r e p r e s e n t t h e i s o t o p i c r a t i o
R i = ( 1 3 C O 2 / 1 2 C O 2 ) , o f c a r b o n d i o x i d e i n s o i l a ir ,
n e t r e s p i r e d C O 2 , a n d a t m o s p h e r i c C O > r e s p e c -
t i v e l y . U s i n g t h e n o t a t i o n :
8~ = ( R ~ 1 ) x 1 0 0 0 f o r 8 s, 8 ~, a n d ~
p D B , 8 )
w h e r e 8 i s t h e p e r m i l v a l u e f o r so i l a i r , t h e a t m o -
sp h e r e , o r r e s p i r e d C O > r e sp e c t i v e l y , a n d R p I) B i s
t h e r a t i o ( 1 3 C / 1 2 C ) i n t h e i s o t o p i c s t a n d a r d P D B .
S u b s t i t u t i n g i n t o e q u a t i o n s ( 4 ) , ( 6 ) , ( 7 ) , a n d ( 8 ) :
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( [ Z 2 . ] t
z 2 D, .
w h e r e :
1 + R p o B ( 1 0 ~ 0 + 1 )
l o g Pc o z
4 .0 3 . 0 2 . 0
c,o~ ~ O~e S
~
_ j ~ , ^ , e
A.
I00
0 ~ 9 Z ~ 1 5 . ~
2 /
~ cp
o o
0 I 2 3 4 5 6
S o i l R e s p i r a t i o n R a t e 1 0 - 3 m o l e s m - 2 h r - i )
Fig. 2. A. Calculated steady state P(CO2) for different soil
respiration rates using the soil model described in the text
where L = 100 cm. B. Calculated steady state carbon isotopic
composition of soil carbon dioxide for different soil respiration
rates using the model described in the text. 3a3C values of
-27%o and -6%o were used for net soil respired CO 2 and
atmospheric COz, respectively.
1000
U s i n g 6 , = - 2 7 a n d 6a = - 6 ~ ( t h e e s t i m a t e d
p r e - i n d u s t r i a l v a l u e f o r a t m o s p h e r i c C O 2 [ 49 ]) i t i s
p o s s i b l e t o c a l c u l a t e t h e i s o t o p i c c o m p o s i t i o n o f
t h e s o i l a t m o s p h e r e . F i g . 2 B s h o w s t h e s t e a d y - s t a t e
d i s t r i b u t i o n o f 6 13 C as a f u n c t i o n o f d e p t h f o r
d i f f e r e n t s o i l r e s p i r a t i o n r a t e s i n t h e m o d e l s o i l
d e s c r i b e d a b o v e . T h e s e c u r v e s a r e c o m p a t i b l e w i t h
t h e f o l l o w i n g o b s e r v a t i o n s : s o il C O 2 i s o f t e n 3 - 7 ~
h e a v i e r t h a n t h e s o i l o r g a n i c m a t t e r [ 7 , 9 , 3 8 , 3 9 ] ;
m o s t s o i l s s a m p l e d d u r i n g p e r i o d s o f h i g h s o i l
r e s p i r a t io n d o n o t s h o w a n i s o t o p i c g r a d i e n t b e l o w
3 0 c m d e p t h [ 9 , 3 9 ] ; d u r i n g p e r i o d s o f l o w s o i l
r e s p i r a t i o n a m e a s u r a b l e i s o t o p i c g r a d i e n t c a n b e
o b s e r v e d [ 40 ]; a n d t h e n e t r e s p i r e d s o i l C O 2 i s
a b o u t 4 ~ l i g h t e r t h a n t h e m e a s u r e d s o i l C O 2 [9 ] .
T h i s l a t t e r s t u d y [ 9] a t t r i b u t e d t h i s e f f e c t t o m o l e c -
u l a r d i f f u si o n . F o r s o il C O 2 c o n c e n t r a t i o n s o f
1 0 2 0, l i m i ts o f - 2 2 . 2 e a n d - 8 . 5 ~ a r e c a l c u -
l a t e d f o r t h e i s o t o p i c c o m p o s i t i o n o f s o il C O 2
d e r i v e d f r o m s o il r e s p i r e d C O 2 w i t h i s o t o p i c c o m -
po s i t ion s of - 27 ~ and - 13 o , re spec t iv e ly , wh ich
r e p r e s e n t p u r e C 3 a n d C 4 b i o m a s s e s , r e s p e c t i v e l y .
T h i s f i g u re S ho w s t h a t t h e a t m o s p h e r i c c o m p o n e n t
i s l i k el y t o b e i m p o r t a n t o n l y w h e n s o il r e s p i ra t i o n
r a t e s a r e q u i t e l o w o r a t v e r y s h a l l o w d e p t h s ( l e s s
t h a n 1 0 c m ) , o r b o t h . T h i s h a s i m p o r t a n t i m p l i c a -
t i o n s c o n c e r n i n g s o i l c a r b o n a t e f o r m a t i o n , w h i c h
w i l l b e d i s c u s s e d b e l o w .
3 .3 . Re la t ionsh ip be tween the carbon o top ic com -
p o s i t i o n o f s o i l ca r b o n a t e a n d t h e p r o p o r t i o n o f
C 4 b ioma ss
T h e c a r b o n i s o t o p i c c o m p o s i t i o n o f s o i l
c a r b o n a t e i s b e s t c o n s i d e r e d w i t h r e s p e c t t o t h e
c a r b o n i s o t o p i c c o m p o s i t i o n o f t h e s o i l a t m o -
s p h e r e. T h e i s o t o p i c c o m p o s i t i o n o f t h e s o il a t m o -
s p h e r e , a s s h o w n a b o v e , i s r e l a t e d t o t h e p r o p o r -
t i o n o f C 4 b i o m a s s , a n d t h e s o i l r e s p i r a t i o n r a t e .
C 4 p l a n t s a r e w e l l a d a p t e d t o c o n d i t i o n s o f h i g h
w a t e r s t re s s , p a r t i c u l a r l y w h e n t h a t s t re s s is r e l a t e d
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t o h i g h t e m p e r a t u r e s . I t h a s b e e n o b s e r v e d t h a t C
g r a s s e s a re n o t p r e s e n t i n f l o r a s w h e n n i g h t t e m -
p e r a t u r e s f a l l b e l o w 8 C [ 32 ,4 7 ]. V e g e t a t i o n i n
r e g io n s o f so il c a r b o n a t e f o r m a t i o n i n c lu d e s m a n y
p l a n t f a m i li e s w h o s e m e m b e r s i n c l u d e b o t h C 3 a n d
C 4 s p e c i e s ( e. g. , P o a c e a e , C h e n o p o d i a c e a e ,
E u p h o r b i a c e a e ) a l th o u g h m o s t t re e s c o m m o n t o
a r i d o r s e m i - a r i d c l i m a t e s a r e C 3 s p e c i e s ( e .g . ,
Acacia . G r a s s e s ( P o a c e a e ) a r e p a r t i c u l a r l y s e n s i -
t i v e t o t e m p e r a t u r e a n d t h e d i s t r i b u t i o n o f C 4
g r a s s e s i s k n o w n f o r m a n y r e g i o n s [ 3 1 , 3 2 , 4 6 , 4 7 ,
5 1 , 5 2 ] . T h e s e k n o w n d i s t r i b u t i o n s w e r e u s e d t o
e s t i m a t e t h e f r a c t i o n o f C 4 fl o r a f r o m t h o s e l o c a l i -
t i es w h e r e g r a s s e s m a k e u p t h e f l o r a ; f o u r l o c a l i ti e s
a r e i n c l u d e d i n t h i s s t u d y w h e r e t h e s e d i s t r i b u t i o n s
c a n n o t b e u s e d : t h e t w o s a m p l e s f r o m t h e l i t e r a -
t u r e (I s r a e l a n d t h e N e t h e r l a n d s ) a n d t h e s a m p l e s
f r o m L a e t o l i a n d N g u u w h i c h a r e i n b l a c k c o t t o n
s o i l s a n d h a v e a s i g n i f i c a n t f r a c t i o n o f
Acacia
drepanolobium
p r e s e n t . B i o m a s s e s t i m a t e s f o r
L a e t o l i a n d N g u u a r e b a s e d o n t h e a s s u m p t i o n
t h a t t h e m e a s u r e d ~ a 3 C v a l u e s o f s o i l o r g a n i c
m a t t e r a r e du e t o t h e g r as s c o m p o n e n t b e c a u s e t h e
s o i l s a m p l e s c o l l e c t e d w e r e s i t u a t e d a w a y f r o m
Acacia t re e s. T h u s t h e v a l u es o f - 1 5 a n d - 1 3 o
r e p r e s e n t g r a s s p o p u l a t i o n s o f 8 5 a n d 1 0 0 C 4
c o m p o n e n t s f o r L a e t o l i a n d N g u u , r e s p e c t i v e l y . I f
t h e Acacia b i o m a s s i s a b o u t 2 5 , t h e n t h e t o t a l C 4
b i o m a s s i s a b o u t 60 a n d 7 5 , r e s p e c t iv e l y , f o r
t h e s e t w o l o c a l i t i e s . B e c a u s e o f t h e u n c e r t a i n t y i n
t h e Acacia b i o m a s s e s t i m a t e , t h e o v e r a l l e s t i m a t e
i n f r a c t i o n C 4 b i o m a s s i s l e ss c e r t a i n f o r t h e s e
l o c a l i t i e s t h a n f o r o t h e r l o c a l i t i e s .
N e w l y - f o r m e d c a l c i t e i n s o i l s c a n p r o b a b l y b e
c o n s i d e r e d t o b e i n i s o t o p i c e q u i l i b r i u m w i t h t h e
g a s p h a s e s i n c e r a t e s o f c h a n g e i n P ( C O 2 ) i n s o i l s
a r e m u c h s l o w e r t h a n t h o s e u s e d i n l a b o r a t o r i e s t o
d e t e r m i n e e q u i l i b r i u m i s o t o p i c f r a c t i o n a t i o n ( e . g . ,
[ 5 3 ] ) . S o m e p r e v i o u s s t u d i e s h a v e c o n s i d e r e d
c a r b o n a t e f o r m a t i o n i n s o i l s t o t a k e p l a c e i n a
c l o s e d s y s t e m [ 18 ,2 0] s o t h a t t h e c a r b o n i s o t o p i c
c o m p o s i t i o n o f t h e s o i l s o l u t i o n c h a n g e s a c c o r d i n g
t o a s i m p l e R a y l e i g h p r o c e s s . T h i s i s u n l i k e l y
b e c a u s e o f t h e c o n t i n u o u s C O 2 f l u x d u e t o s o i l
r e s p i r a t i o n . T h u s , s o i l s o l u t i o n s a r e c o n s i d e r e d t o
b e i n e q u i l i b r i u m w i t h a C O 2 r e s e r v o i r w h o s e
i s o t o p i c c o m p o s i t i o n i s u n c h a n g e d b y c a r b o n a t e
f o r m a t i o n .
F o r t h e p u r p o s e o f d i s c u s s i n g t h e g l o b a l d i s t r i -
b u t i o n o f 61 3C i n s o il c a r b o n a t e , a n i s o t o p i c f r a c -
t i o n a t i o n ( 1 0 3 i n a ) o f - 1 0 . 3 6 [ 5 4 ] a t 2 5 C f o r
C O 2 - C a C O 3 w i ll b e u s e d . U s i n g t h is t o c a l c u l a t e
t h e i s o t o p i c c o m p o s i t i o n o f s o i l c a r b o n a t e i n e q u i -
l i b r iu m w i t h t h e C O 2 o f t h e p u r e c o m p o n e n t e n d -
m e m b e r s , i t is p o s s i b l e to s h o w t h e m i x i n g o f t h e s e
r e s e r v o i r s . F i g . 3 s h o w s t h i s m i x i n g a n d g i v e s t h e
6 1 3 C v a l u e s f o r m o d e r n s o i l c a r b o n a t e a n a l y z e d i n
t h i s s t u d y ; i t s h o w s t h a t i n g e n e r a l t h e r e i s g o o d
a g r e e m e n t b e t w e e n t h e c a r b o n i s o t o p i c c o m p o s i -
t i o n o f s o i l c a r b o n a t e a n d t h e f r a c t i o n o f C 4 b i o -
m a s s p r e s e n t . T h e h i g h s t a n d a r d d e v i a t i o n f o r t h e
s a m p l e s f r o m I o w a r e s u l t f r o m t h e p r e s e n c e o f tw o
i s o t o p i c p o p u l a t i o n s w i t h 6 1 3 C v a l u e s o f a b o u t
- 2 .9 0 an d - 7 .2 0 , r e sp ec t iv e ly .
[
5 i o o
- 1
- I 0 -
1 5 1 0 I L I i 1 t I
. 5 0
F R A C T I O N C 4 F L O R A
F i g . 3. C a r b o n i s o t o p i c c o m p o s i t i o n o f s oi l c a r b o n a t e c o m -
p a r e d t o e s t i m a t e o f t h e f r a c t i o n o f C 4 p l a n t s i n l o c a l f l o r a
( d a t a f r o m T a b l e 1 ), M i x i n g l i n e s o f c a l c i te i n e q u i l i b r i u m w i t h
s o i l C O 2 a n d a t m o s p h e r i c C O 2 s h o w n f o r r e fe r e n c e . T h e s e a r e
c a l c u l a t e d u s i n g a n i s o t o p ic f r a c t i o n a t i o n f a c t o r ( 1 0 3 I n a ) o f
- 1 0 . 3 6 o a t 2 5 C , a n d u s i n g ~ 1 3 C v a l u e s o f - 22 .2 0 , - 8 .5 ~ ,
a n d - 6 o f o r t h e c a r b o n i s o t o p i c c o m p o s i t i o n f o r s o il C O 2
f r o m a 10 0 C 3 f l o r a , s o i l C O 2 f r o m a 1 0 0 C 4 f l o r a , a n d
a t m o s p h e r i c C O 2 , r e s p e c t i v e ly . N u m b e r s r e f e r t o l o c a li t y n u m -
b e r s i n T a b l e 1 .
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3 4 Impl icat ions for soi l carbonate formation
S o i l c a r b o n a t e f o r m a t i o n i s g e n e r a l l y c o n s i d -
e r e d t o r e s u l t f r o m c a r b o n a t e s u p e r s a t u r a t i o n d u e
t o e v a p o r a t i o n , e v a p o t r a n s p i r a t i o n , a n d l o w e r i n g
o f P ( C O 2 ) [ 1 , 2 , 1 7 - 2 1 , 2 8 ] . O f t h e s e , e v a p o r a t i o n i s
p r o b a b l y o f l i t t l e s i g n i f i c a n c e b e c a u s e e v a p o t r a n -
s p i r a t io n i s t h e d o m i n a n t m e c h a n i s m f o r s o il w a t e r
l o s s in a r e a s c o v e r e d b y v e g e t a t i o n [ 2 ,5 5] . F e w
s t u d ie s h a v e e x a m i n e d t h e e f f e c t o f e v a p o t r a n s -
p i r a t i o n o n t h e i s o t o p i c c o m p o s i t i o n o f s o i l s o l u -
t i o n s b u t i t i s t h o u g h t t o b e n o n - f r a c t i o n a t i n g
[ 4 , 5 7 ] . W i t h o u t d e t a i l e d s t u d i e s o f t h e a n n u a l
o x y g e n a n d h y d r o g e n i s o t o p i c v a r i a t i o n s i n s o i l
w a t e r s , i t i s n o t p o s s i b l e t o a d d r e s s t h i s p r o b l e m .
T h e c a r b o n i s o t o p i c c o m p o s i t i o n o f t h e s e s o i l
c a r b o n a t e s d o e s h a v e i m p o r t a n t i m p l i c a t i o n s c o n -
c e r n i n g s o i l c a r b o n a t e f o r m a t i o n . O f p a r t i c u l a r
i n t e r e s t a r e t h o s e s a m p l e s t h a t f a l l a b o v e t h e 1 0 %
a t m o s p h e r i c c o m p o n e n t l i n e . S a m p l e s b e l o w t h e
l i n e ( e x c e p t I o w a w h i c h f a ll s n e a r t h e 1 0 % l in e ) a r e
f r o m r e g i o n s w h e r e t h e s o il d o e s n o t f r e e z e t o t h e
d e p t h o f c a r b o n a t e f o r m a t i o n ; a ll sa m p l e s w i t h a
s i g n i f i c a n t a t m o s p h e r i c c o m p o n e n t a r e f r o m r e -
g i o n s w h e r e t h e s o i l a n n u a l l y f r e e z e s t o t h e d e p t h
o f c a r b o n a t e f o r m a t i o n . T h i s i m p l i e s t h a t s o m e
c a r b o n a t e m a y b e f o r m e d d u r i n g p e r io d s o f l ow
s o il r e s p ir a t i o n r at e s. A n i m p o r t a n t m e c h a n i s m f o r
s o i l c a r b o n a t e f o r m a t i o n c o u l d b e d u e t o t h e i n -
c r e a s e i n i o n c o n c e n t r a t i o n s t h a t r e s u l t s f r o m i o n
e x c l u s i o n d u r i n g i c e f o r m a t i o n . B e c a u s e s o i l r e s p i -
r a t i o n r a t e s a r e o n t h e o r d e r o f 0 . 2 5 1 0 3 m o l e s
m 2 h r - 1 o r l es s d u r i n g p e r i o d s o f s o il fr e e z i n g
(e .g . , [43] ) i t i s pos s ib le to ge t a h igh a tmospher i c
c o m p o n e n t i n s o i l c a r b o n a t e f o r m e d b y t h i s p r o -
c e s s ( F i g . 2 B ) . A s i n g l e s a m p l e o f c e m e n t e d d u n e
m a t e r i a l f r o m A l a s k a [ 2 9 ] w a s e x a m i n e d w i t h t h i s
in min d; i t y i e ld ed a 813C va lu e o f + 1.1 _+ 1 .1%o
i n d i c a t i v e o f a h i g h a t m o s p h e r i c c o m p o n e n t . T o
c a l c u l a t e t h e a t m o s p h e r i c c o m p o n e n t f o r s u c h a
s a m p l e o n e w o u l d o f c o u r s e h a v e t o c o m p u t e t h e
a t m o s p h e r i c c o m p o n e n t b a s e d o n a t e m p e r a t u r e o f
0 C r a t h e r t h a n 2 5 C a s i n F i g . 3 . T h i s r e s u l t s i n
a n u p p e r l i m i t o f a b o u t + 8% o f o r t h e i s o t o p i c
c o m p o s i t i o n o f s o i l c a r b o n a t e p r e c i p i t a t e d a t 0 C
f r o m a 1 00 % a t m o s p h e r i c c o m p o n e n t . I f a p p l i e d
t o t h i s s a m p l e f r o m A l a s k a , t h i s i m p l i e s a n a t m o -
s p h e ri c c o m p o n e n t o f 6 0 - 8 0 % .
235
S t u d i e s o f s o i l c a r b o n a t e f r o m s i n g l e s o i l s o f t e n
h a v e a d e v i a t i o n o f a b o u t _ + 0. 5% f o r 6 1 3 C ( e. g .,
[ 1 8 , 2 0 ] ) . S o i l c a r b o n a t e f o r m e d b y s o l u t i o n c o m -
p o s i t i o n c h a n g e s r e s u l t i n g f r o m e v a p o t r a n s p i r a -
t i o n , f r o m c h a n g e s i n P CO2 ) , o r r e s u l t i n g f r o m
s o il s o l u t i o n f r e e z i n g h a v e i s o t o p i c s i g n a t u r e s t h a t
r e s u l t f ro m t h e t h r e e - c o m p o n e n t m i x i n g d e s c r i b e d
a b o v e . T h i s m o d e l s u g g e s t s t h a t v a r i a t i o n s i n t h e
8 ~ 3 C o f s o i l c a r b o n a t e m a y r e s u l t f r o m m i x i n g o f
t h e s e t h r e e r e s e r v o i r s a n d f r o m v a r i a t i o n s i n t h e
p r o p o r t i o n o f C 4 f lo r a s s e a s o n a ll y o r o v e r a l o n g
i n t e r v a l o f t i m e . P r e v i o u s l y , t h e s e c h a n g e s h a v e
b e e n a t t r i b u t e d t o c l o s e d s y s t e m c a r b o n a t e f o r m a -
t i o n [ 1 9 , 2 0 ] o r t o c o n t a m i n a t i o n w i t h d e t r i t a l
materia l [18,22].
3 5 Relationship between 6180 and 613C in soi l
carbonate
A c o u p l i n g i s e x p e c t e d b e t w e e n t h e 8 1 3 C a n d
6 ~S O v a l u e s f o r m a n y s o i l c a r b o n a t e s . T h e f r a c t i o n
o f C 4 g ra s s e s i s w e l l c o r r e l a t e d w i t h n i g h t - t i m e
t e m p e r a t u r e s [ 1 0 , 3 2] ; h e n c e , t h e 8 13 C v a l u e f o r s o i l
C O 2 is e x p e c t e d t o b e h i g h e r i n r e g i o n s w i t h h i g h
n i g h t t e m p e r a t u r e s . T h e o x y g e n i s o t o p i c c o m p o s i -
t i o n o f m e t e o r i c w a t e r s f r o m c o n t i n e n t a l s t a t i o n s
r e c e i v i n g l es s t h a n 1 0 0 0 m m a n n u a l p r e c i p i t a t i o n
i s w e l l c o r r e l a t e d w i t h m e a n a n n u a l t e m p e r a t u r e
[ 3 5 ] e x c e p t f o r r e g i o n s w h i c h h a v e m o n s o o n a l
c l i m a t i c c o n d i t i o n s , w h i c h a r e o f t e n 4 - 6 % o d e -
p l e t e d i n ~ 8 0 r e l a t i v e t o c o n t i n e n t a l s t a t i o n s o f
s i m i l a r t e m p e r a t u r e s ( F i g . 4 ) . I n m o s t r e g i o n s , t h e
n i g h t - t e m p e r a t u r e s d u r i n g t h e g r o w i n g s e a s o n a n d
m e a n a n n u a l t e m p e r a t u r e a r e w e l l c o r r e l a t e d ; a n
i m p o r t a n t e x c e p t i o n a r e t h o se c li m a t e s b u f f e r e d b y
m a r i n e c o n d i t i o n s w h i c h a r e e x p e c t e d t o h a v e a
d i s p r o p o r t i o n a t e l y h i g h p e r c e n t a g e o f C 3 p la n t s .
T h u s , S a n F r a n c i s c o (U . S . A . ) a n d S t . L o u i s
( U . S . A . ) b o t h h a v e m e a n a n n u a l t e m p e r a t u r e s o f
1 3 C , b u t h a v e J u ly m i n i m u m t e m p e r a t u r e s o f 1 2
a n d 1 9 C , a n d C 4 / ( C 3
+ C4
p e r c e n t a g e s o f 8 %
an d 60%, re spec t ive ly [32].
F i g . 5 B s h o w s a g e n e r a l m o d e l f o r t h e i s o t o p i c
c o m p o s i t i o n o f s o i l c a r b o n a t e s d e v e l o p e d o n g r as s -
l a n d s. N o r m a l c o n t i n e n t a l s o il c a r b o n a t e s a r e
r e p r e s e n t e d i n t h e s h a d e d a r e a , w h i c h i s b o u n d e d
b y t h e 0 - 3 0 % a d m i x t u r e o f t h e a t m o s p h e r i c c o r n -
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2 3 6
0
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I I I I I I [ [
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I n t e n s e , h e a v y r a i n f a l l s o f t e n s h o w a d e p l e t i o n i n
1 8 0 ; m o n s o o n a l w a t e r s i n I n d i a a r e d e p l e t e d b y
a b o u t 6 0 r e l a t i v e t o n o r m a l c o n t i n e n t a l w a t e r s
( F i g . 4 ) . C a l c r e t e s f r o m I n d i a [ 2 0 ] s h o w t h i s e f f e c t
a n d f a l l i n f i e l d C i n F i g . 5 B . A n o t h e r i m p o r t a n t
s i t u a t i o n m a y b e p r e s e n t i n p e r i g l a c i a l e n v i r o n -
m e n t s ; i f c a r b o n a t e p r e c i p i t a t io n r e su l ts p r i m a r i l y
f r o m s o il f r e e z i n g w h e n s o il r e s p i r a t i o n r a t e s a r e
l o w , v e r y h i g h 6 13 C v a l u e s a r e p o s s i b l e ( f i e ld D i n
F i g . 5 B ) . O n e s a m p l e i n o u r s t u d y f r o m A l a s k a
m a y b e r e p r e s e n t a t i v e o f t h i s c o n d i t i o n ; D e v e r e t
a l. [ 22 ] h a v e a l s o i d e n t i f i e d s o il c a r b o n a t e t h a t t h e y
a t t r i b u t e t o p e r i g l a c i a l c o n d i t i o n s . T h e p r e s e n c e o f
a s i g n i f i c a n t n o n - g r a s s c o m p o n e n t i n t h e b i o m a s s
w o u l d a l s o t e n d t o m a k e 3 1 3 C v a l u e s m o r e n e g a -
t i v e ; t h i s w a s i m p o r t a n t i n t w o o f t h e s a m p l e s
s t u d i e d .
3 6 Use of the isotopic com posi t ion of soi l carbon ate
as a pa leoc l imat i c ind ica tor
P a l e o s o l c a r b o n a t e s m a y b e u s e f u l i n s o m e c a s e s
i n d e t e r m i n i n g p a l e o c l i m a t e s s i n c e , i f t h e y a r e
u n a l t e r e d b y d i a g e n e s i s , t h e y r e c o r d i n f o r m a t i o n
c o n c e r n i n g b o t h t h e i s o t o p i c c o m p o s i t i o n o f
m e t e o r i c w a t e r a n d t h e p r o p o r t i o n o f C 4 b i o m a s s
p r e s e n t i n t h e e c o s y s te m . T w o p r o b l e m s a r e o f
i m m e d i a t e c o n c e r n : o v e r p r i n t i n g a n d d i ag e n e si s.
O v e r p r i n t i n g r e f e r s t o t h e i m p o s i t i o n o f l a t e r
c l i m a t ic i n f o r m a t i o n o n p r e v i o u s i n f o r m a t i o n . T h i s
c o u l d t a k e p l a c e i f t h e s o i l c a r b o n a t e i n a s i n g l e
h o r i z o n f o r m e d d u r i n g s e v e r a l d i f f e r e n t c l i m a t i c
e p i s o d e s . I t i s p o s s i b l e t h a t s o m e o f t h e v a r i a t i o n
o b s e r v e d i n s o i l c a r b o n a t e s , e s p e c i a l l y c a l c r e te s , i s
d u e t o t h i s p r o b l e m . I n e v a l u a t i n g t h i s a s a p o t e n -
t i a l p r o b l e m , o n e c a n c o n s i d e r t h e r e l a t i o n s h i p
b e t w e e n t h e z o n e o f s o i l c a r b o n a t e f o r m a t i o n a n d
t h e s e d i m e n t a t i o n r a t e . S o il c a r b o n a t e h o r i z o n s a r e
g e n e r a l l y l es s t h a n o n e m e t e r t h i c k ; p e r i o d s o f
c l i m a t i c c h a n g e a r e o n t h e o r d e r o f 1 0 , 0 0 0 y e a r s o r
m o r e . S t u d i e s o f t e r r e s t r i a l s e q u e n c e s i n A f r i c a [ 2 5]
a n d N o r t h A m e r i c a [ 5 8 ] s h o w t h a t i n t e r v a l s o f
c l i m a t i c c h a n g e m a y b e r a p i d ( < 5 0 , 0 0 0 y e a r s ) b u t
l o n g i n t e r v a l s o f r e l a t i v e l y c o n s t a n t c l i m a t i c c o n d i -
t i o n s ( 4 0 0 , 0 0 0 y e a r s ) m a y p e r s i s t . S e d i m e n t a t i o n
r a t e s m u s t t a k e t h i s i n t o a c c o u n t : v e r y lo w s e d i-
m e n t a t i o n r a t e s ( le ss th a n 1 c m / 1 0 0 0 y e a r s ) m a y
h a v e a n i s o t o p i c r e c o r d t h a t s p a n s s e v e r a l d i f f e r e n t
237
c l i m a t i c r e g i m e s ; t h i s i s p r o b a b l y t h e c a s e f o r
m a n y c a lc r e te s . H i g h e r s e d i m e n t a t i o n r a t e s ( g r e a t er
t h a n 1 0 c m / 1 0 0 0 y e a r s ) m a y r e s u l t i n t h e p r e -
s e r v a t i o n o f s o i l c a r b o n a t e t h a t r e t a i n s c l i m a t i c
i n f o r m a t i o n o f a l i m i t e d t i m e s p a n .
D i a g e n e t i c e f f e c t s a r e a l s o n o t e a s i l y e v a l u a t e d
a t t h i s t i m e . T h e c a r b o n a t e p r e c i p i t a t i o n a n d p a r -
t i a l r ed i s so lu t ion in so i l s i s mos t s ens i t ive in the A
a n d B h o r i z o n s o f s o i l s ; c h a n g e s i n P ( C O 2 ) a n d
w a t e r l o s s d u e t o e v a p o t r a n s p i r a t i o n a r e m i n i m a l
b e l o w t h e B h o r i z o n . T h e p r e s e n c e o f m i c r i t e i n
p a l e o s o l s i n d i c a t e s t h a t m i n i m a l r e c r y s t a l l i z a t i o n
h a s o c c u r r e d . C o - e x i s ti n g m i c r i te a n d s p a r it e f r o m
b u r i e d p a l e o s o l n o d u l e s i n E o c e n e s e d i m e n t s i n
W y o m i n g s h o w e d t h a t 8 ~ 3 C v a l u e s a r e e s s e nt i al ly
u n c h a n g e d b y r e c r y s ta l l iz a t i o n b u t 8 1 S O v a l u e s
w e r e o f t e n 4 - 8 ~ d e p l e t e d i n 1 8 0 in t h e s p a r i t e
( u n p u b l i s h e d d a t a ) . P e d o g e n i c c a r b o n a t e s f r o m
O l d u v a i G o r g e s h o w a n e x c e l le n t r e l a t io n s h i p w i t h
t i m e [ 25 ]. T h e p e r s i s t e n c e o f 6 1 3C a n d 6 X SO v a l u e s
n o t c o m p a t i b l e w i t h t o d a y ' s c l i m a t e , a n d t h e e x c e l -
l e n t c o r r e s p o n d e n c e b e t w e e n 1 4 C a g e s o n c a l c r e t e s
a n d o n a s s o c i a t e d o r g a n i c m a t e r i a l a t O l d u v a i
G o r g e [ 2 5 ] i s e v i d e n c e t h a t i n s o m e c a s e s d i -
a g e n e t i c a l t e r a t i o n d o e s n o t a p p e a r t o b e i m -
p o r t a n t .
4 . o n c l u s i o n s
T h i s s t u d y i n d i c a te s t h a t t h e o x y g e n a n d c a r b o n
i s o t o p i c c o m p o s i t i o n o f s o il c a r b o n a t e i s r e l a te d t o
t h e i s o t o p ic c o m p o s i t i o n o f m e t e o r i c w a t e r a n d t o
t h e p r o p o r t i o n o f C 4 b i o m a s s p r e s e n t. B e c a u s e o f
t h i s , s o i l c a r b o n a t e c a n b e a n i m p o r t a n t p a l e o c l i -
m a t i c a n d p a l e o e c o l o g i c i n d i c a t o r . B e c a u s e t h e
p r o p o r t i o n o f C 4 b i o m a s s is r e l at e d t o t e m p e r a t u r e
a n d b e c a u s e t h e o x y g e n is o t o p i c c o m p o s i t i o n o f
m e t e o r i c w a t e r is r e l a t e d t o t e m p e r a t u r e , a p o s i t i v e
c o r r e l a t i o n b e t w e e n 6 1 3 C a n d 6 1 8 0 i n s o i l
c a r b o n a t e s i s s o m e t i m e s f o u n d . M o n s o o n a l ,
c o a s t a l , a n d p e r i g l a c i a l c l i m a t e s h a v e d i f f e r e n t 6 13 C
a n d 8 1 S O r e l a ti o n s h i p s b e c a u s e o f d i f f e r e n c e s in
t h e i s o t o p i c c o m p o s i t i o n o f m e t e o r i c w a t e r s i n
m o n s o o n s , i n f l u e n c e s o f t h e o c e a n s , a n d l o w s o i l
r e s p i r a t i o n r a t e s , r e s p e c t i v e l y .
I n a d d i t i o n , t h i s a n a l y s i s i n d i c a t e s t h a t t h e o b -
s e r v e d d i f f e r e n c e s i n 8 1 3 C c o n t e n t s o f s o i l
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238
c a r b o n a t e s i s n o t d u e t o c a r b o n a t e f o r m a t i o n i n a
c l o s e d s y s t e m , b u t r a t h e r t h a t d i f f e r e n c e s i n t h e
i s o t o p i c c o m p o s i t i o n o f s o il C O 2 c a n r e s u l t f r o m
c h a n g e s i n t h e s oi l r e s p i r a t i o n r a t e a n d c h a n g e s i n
t h e p r o p o r t i o n o f C 4 b i o m a s s s e a s o n a l l y o r o v e r a
l o n g t i m e i n te r v a l . T h e o b s e r v e d d i f f e r e n c e b e -
t w e e n t h e i s o t o p i c c o m p o s i t i o n o f s o il c a r b o n d i -
o x i d e a n d t h e a s s o c i a t e d o r g a n i c m a t t e r i n s o il i s
e x p e c t e d b e c a u s e o f t h e d i f f e r e n c e i n d i f fu s i o n
c o e f f i c i e n t s f o r 3COz and 1 2 C O 2. I n a d d i t i o n , t h e
p r e s e n c e o f a h i g h a t m o s p h e r i c c o m p o n e n t i n s o m e
s o il c a r b o n a t e s s u g g e s t s t h a t f r e e z i n g o f s o il s o l u -
t i o n s m a y b e a n a d d i t i o n a l c a r b o n a t e f o r m a t i o n
m e c h a n i s m .
cknowledgements
G . W . C o x , S . G r e e n , R i . H a y , J . L . R i c h a r d -
s o n , a n d R . J . S t. A r n a u d p r o v i d e d s o m e o f t h e
m o d e r n c a r b o n a t e s u s e d in th i s s t u d y ; T . M . B o w n
p r o v i d e d a s s i s t a n c e in t h e f ie l d ; J . R . B o w m a n a n d
R . L a m b e r t p r o v i d e d l a b o r a t o r y a s s i s t a n c e . I t h a n k
F . H . B r o w n , M . C . M o n a g h a n , a n d W . T . P a r ry fo r
m a n y h e l p f u l d i s c u s s i o n s . R e v i e w e r s i m p r o v e d t h e
c l a r i t y o f th i s m a n u s c r i p t . T h i s w o r k ( i n c l u d i n g
p a l e o s o l s ) w a s s u p p o r t e d b y t h e L . S .B . L e a k e y
F o u n d a t i o n , t h e F o u n d a t i o n f o r R e s e a r c h i n t h e
O r i g i n o f M a n , t h e N a t i o n a l S c i e n c e F o u n d a t i o n
( B N S - 8 0 0 7 3 5 4 a n d B N S - 8 2 1 0 7 3 5 ) , a n d t h e U . S .
G e o l o g i c a l S u r v e y ( g r a n t to T .M . B o w n ) .
ppendix I
(1) Olduvai Gorge Tanzania: 1.5 c m th ic k l a m ina r c a lc re te
f o r m e d i n a e o l i a n t u ff p r i o r t o d e p o s i t io n o f t h e N a m o r o d A s h
[ 2 3 - 2 5 ]. C a l c r e t es a t O l d u v a i G o r g e h a v e b e e n s h o w n t o f o r m
ve ry quic kly be c a use of the i r h ighly r e a c t ive pa re nt m a te r ia l
[24] . Dom ina nt gra s s spe c ie s inc lude Digitaria macroblephara
a n d Sporabolus f imbriatus [ 2 6 ] . O l d u v a i G o r g e h a s a m e a n
a n n u a l t e m p e r a t u r e o f a b o u t 2 3 C .
(2) Laetoli Tanzania: 1 c m n o d u l e s f r o m b l a c k c o t t o n s o il
(ve r t iso l ) . Thi s s i t e is on the e dge of the Se re nge t i P la in b ut i s
5 - 6 C c o o l e r t h a n O l d u v a i G o r g e b e c a u s e o f i ts h i g h e r e le v a -
t ion of a bo ut 1800 m . Ve g e ta t ion i s gra s s la nd wi th som e Ac ac ia
drepanolobium the whis t l ing thorn .
(3) Nguu Kenya: 1 m m n o d u l e s f r o m b l a c k c o t t o n s o i l
(ve r t i so l ) . Ve ge ta t ion i s gra s s la nd wi th som e Acacia drepano-
lobiurn. M e a n a n n u a l t e m p e r a t u r e i s a b o u t 2 3 C .
(4) The Netherlands: S a l o m o n s a n d M o o k [ 1 9 ] r e p o r t e d o n
loe s s nodule s f rom the Ne the r la nds . The a ge of nodule form a -
t ion i s La te P le i s toc e ne or Holoc e ne .
(5) Israel: Ma g a r i t z e t a l . [18] ha ve s tudie d so i l c a rbon a te s
f rom the c oa s ta l p la in of I s r a e l . Sa m ple s quote d in th i s s tudy
inc lude only those so i l c a rbona te s tha t ha d 14C a ge s l e s s tha n
10,000 B.P.
(6) Iowa U.S.A.: Loe ss nodule s f rom the oxid iz e d a nd
unle a c he d z one of Wisc ons in loe s s we re c o l l e c te d f rom Loga n,
I o w a . C a r b o n a t e c o n t e n t o f t h e p a r e n t m a t e r i a l i s a b o u t 5
Ca C O 3 [27] , bu t the loe ss nodule s a pproa c h 100 c a lc it e . Loe ss
de pos i t ion c e a se d a bout 14 ,000 B .P . in Iowa [27] ; s inc e the se
nodule s we re f rom high in the loe s s prof i l e , i t i s p re sum e d tha t
the y a re of a bout th i s t im e or l a te r . Argum e nts of Ruhe [27]
s h o w t h a t t h e o x i d i z e d a n d u n l e a c h e d z o n e m u s t h a v e f o r m e d
before 6800 B.P. Boute loua Andropogon Agropyron a n d St ipa
a re im por ta n t e le m e nts of th i s pra i r i e f lora [10 ,32] . Thi s a re a
h a s a m e a n a n n u a l t e m p e r a t u r e o f a b o u t 1 1 C .
(7) North Dakota U.S.A. : The Wi l l i a m s so i l s e ri e s c onta ins
1 m m soi l c a rbona te nodule s ; th i s so i l i s form e d on Wisc ons in
gla c ial ti l l tha t ha s a bout 5 de t r i t a t c a rbo na te in the pa re n t
m a te r ia l . Thi s i s in the m ixe d-gra s s pra i r i e of c e nt r a l Nor th
A m e r i c a ; i t h a s a m e a n a n n u a l t e m p e r a t u r e o f a b o u t 5 C .
(8) Saskatchewan Canada: Sa m ple s we re t a ke n f rom thre e
di f f e re nt so i l s in c e nt r a l Sa ska tc he w a n; de ta i l s of the se so i l s a re
d i sc usse d in S t . Arn a ud [28] . The p a re nt m a te r ia l c onta in s a
fe w pe rc e nt de t r i t a l c a rbona te ; 14C da te s on d i f f e re nt so i l s i z e
f r a c t ions show d e c re a s ing a ge wi th d e c re a s ing pa r t i c le s iz e . The
le s s tha n 2 /~ m s iz e f r a c t ion y ie lds 14C a ge s of 1595 -7200 B .P .,
whe re a s the c oa r se s i l t a nd sa nd s iz e f r a c t ion y ie ld a ge s of
> 30,000 B.P. On ly car bo nate s in the less than 2 ~tm s ize
f ra c t ion a re r e por te d he re . A l l so i l s we re form e d on g la c ia l
de pos i t s . Thi s pra i r i e s i t e ha s a m e a n a nnua l t e m pe ra ture of
a b o u t 2 C .
(9) Alaska U.S.A.: C a r b o n a t e c e m e n t e d s a n d d u n e s h a v e
b e e n r e p o r t e d a t l o n g i t u d e 1 5 8 W a n d l a t i t u d e 6 7 N [ 2 9 ]. S a n d
f rom the se dune s doe s not ha ve c a lc i t e pre se nt , a l though a
s ta b i l i z e d dune f i e ld som e 25 km di s ta n t i s r e por te d to ha ve
s o m e d e t r i ta l c a r b o n a t e p r e s e n t [ 2 9 ] . C a r b o n a t e f o r m a t i o n i s
t h o u g h t t o h a v e t a k e n p l a c e s in c e o r d u r i n g l a t e W i s c o n s i n t i m e
[ 29 ]. E s ti m a t e d m e a n a n n u a l t e m p e r a t u r e is a b o u t - 7 C .
(10) Utah U.S.A.: L a m i n a r c a l c r e t e c o a t i n g b o u l d e r n e a r
T i n t i c , U t a h . T h i s d e s e r t g r a ss l a n d h a s a m e a n a n n u a l t e m p e r a -
t u r e o f a b o u t 7 C .
(11) Wyoming U.S.A. I : L a m i n a r c a l cr e t e c o at i n g s o n p e b -
b le s in pos t g la c ia l f i l l o r a l luv ium a t a bout 2500 m e le va t ion .
Pe bble s we re a bout 20 c m be low the sur fa c e . Thi s a re a ha s a n
e s t i m a t e d m e a n a n n u a l t e m p e r a t u r e o f a b o u t 4 C . T h i s i s a n
a lp ine gra s s la nd .
(12) H@oming U.S.A. 11: La m ina r c a lc re te c oa t ings on
o r t h o q u a r t z i t e p e b b l e s o n p o s t g l a c i a l a l l u v i u m a t a b o u t 2 0 0 0
m e t e r s e l ev a t io n . E s t i m a t e d m e a n a n n u a l t e m p e r a t u r e i s a b o u t
6C.
(13) Paleosol carbonates: Pa le osol c a rbona te s f rom bur ie d
s o i l s i n E o c e n e s e d i m e n t s i n W y o m i n g ( U . S . A . ) a n d f r o m
Pl ioc e ne a nd P le i s toc e ne se dim e nts in Ke nya , Spa in , a nd
Ta nz a nia a l so ha ve be e n a na lyz e d . O the r pa le osol a nd so i l
c a rbo na te s f rom l i t e r a ture sourc e s a re d i sc usse d a s we l l.
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